1. Field of the Invention
The present invention relates to a sinusoidal wave generation apparatus for generating a sinusoidal wave signal of a precise frequency.
2. Description of the Prior Art
In conventional sinusoidal wave generation apparatus, a sinusoidal wave signal is generated using an analog method in which an operational amplifier and a passive element are combined, or using a digital/analog conversion method in which sine wave data is stored in a read only memory (ROM) and then converted into an analog signal at a fixed period in response to a clock pulse.
However, such a conventional sinusoidal wave generation apparatus using the analog method has a disadvantage in that it is difficult to vary an amplitude or a frequency of the sinusoidal wave signal in a circuitry of a fixed construction because the amplitude or the frequency of the sinusoidal wave signal is determined according to a value of the passive element. Also, in the case where the frequency of the sinusoidal wave signal is intended to become precise, a precision of the passive element is required. This results in an increase in the cost. Further, it is hard to use the passive element in generating the sinusoidal wave signal of the precise frequency since the value of the passive element is sensitive to an variation in circumstances such as temperature, humidity, etc.
Also, another conventional sinusoidal wave generation apparatus using the digital/analog conversion method is desirable to generate the sinusoidal wave of a precise frequency since the clock frequency is precise, but has the disadvantage that the ROM is required and the amplitude of the sinusoidal wave signal cannot be varied because the sinusoidal wave signal is generated on the basis of the sine wave data stored in the ROM. One example of this form of conventional sinusoidal wave generation apparatus is disclosed in Japanese Utility Model Laid-open Publication No. Heisei 1-133200 and is shown in FIG. 1, herein.
As shown in FIG. 1, the conventional sinusoidal wave generation apparatus using the digital/analog conversion method comprises an oscillator 1 for generating a square wave signal, a frequency divider 2 for dividing the square wave signal from the oscillator 1 into a plurality of frequency signals, a selector 3 for selecting one of the plurality of frequency signals from the frequency divider 2, a counter 4 for counting a pulse of the frequency signal selected by the selector 3, a programmable read only memory (PROM) 5 for outputting sinusoidal wave data stored therein in response to an output signal from the counter 4, and a digital/analog (D/A) converter 6 for converting the output, data from the PROM 5 into an analog signal. However, this conventional sinusoidal wave generation apparatus has the above-mentioned problems.